SOIL PROPERTIESA crash course in soils

D. SmithRaleigh Charter H.S.

2005-06

What is Soil?? Soil is the top layer of the Earth’s surface. Soils are the transition between the biotic

and abiotic worlds. Soil is a mixture of:

1. Minerals 2. Water3. Gases4. HUMUS-Dead “things” that have broken down

and become organic material—decayed plant and animal remains

(thanks to decomposing fungi and bacteria)

Soil Composition Mineral matter

comes from parent material

Air & Water percents are interchangeable

Humus is organic matter!

Water (~25%)

Air (~25%)

Mineral(~45%)

Humus (~5%)

Soils are important:◦ As a habitat for growing crops

Food Fabrics/Dyes Rubber/Building Materials

◦ As foundations of buildings◦ As beds for roads and highways◦ As an absorbent of domestic wastes in rural

areas as well as a depository for other wastes◦ As a filter for pollution that comes from rain and

water runoff◦ Medium for photosynthesis

Why are soils important?

There are THREE main contributing factors

1. The materials (rocks) from which they form- these are called PARENT MATERIAL

2. The ENVIRONMENT in which they developed (climate, vegetation, soil life, topography)

3. Length of time soils have been developing—the AGE of the soils

Good rule of thumb: 500-1000 yrs per inch of topsoil

How does soil form?

Mineral Matter◦ Provides nutrient minerals

for plants◦ Provides pore space for

water & air

Older soils are:◦ More weathered◦ Lower in certain

essential nutrient minerals

Soil Composition continued…

Humus: black/dark brown organic material remaining after

decomposition– Leaf litter, animal dung, & decaying organisms– Increases the soil’s water-holding capacity by

acting like a sponge

WEATHERING: Any process where rock breaks down (changes chemically and physically)

There are two types of weathering:Physical or Mechanical Weathering: Large

rock mass is broken into smaller fragments of the same type-NO CHEMICAL CHANGE

Chemical Weathering: mass of rock is decomposed by chemical reactions

How do parent materials get into soils??

Followed by Erosion (Wind, Water, Ice) What are “agents” of physical weathering?

◦ Tree roots/plant roots◦ Frost wedging:

1. Water collects in cracks in rock2. Water expands when it freezes3. Pieces of rock break off

Physical Weathering

Let’s take a closer look at frost wedging…

1.Water collects in cracks in rock

2.Water expands when it freezes

3.Water melts; Pieces of rock break off

Rock

Most chemical weathering involves gases in the atmosphere (oxygen and carbon dioxide) and/or water

Example: Think about metal rusting◦You go from metal to rust—a chemical reaction has taken place

Chemical Weathering

Topsoil Formation

Detritus-based Soil Ecosystem

All these happy little critters break down dead plant and animal

material and aerate the soil—they make topsoil!

Earthworms & Ants: ◦ Cycle minerals◦ Aerate the soil◦ Decomposing corpses

contribute organic matter◦ Assist plants in

reproduction by burying seeds

Mycorrhizae◦ Symbiotic relationship

between fungi and the roots of vascular plants

◦ Fungus absorbs nutrients from the soil, and provides it for the plant.

◦ Plant produces food (photosynthesis), and provides it for the fungus.

Soil Food Web

Soil Food Web (incl. humans)

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Texture pH Permeability Porosity Nutrient Levels—Nitrogen/Phosphorus

What are some soil properties?

Soil texture: The way a soil "feels"◦ Depends on the fraction each size of

particle in the soil◦ Sand, silt, and clay are names that

describe the size of individual particles in the soil. SAND are the largest particles and they feel

"gritty.“ SILT are medium sized, and they feel soft,

silky or "floury" CLAY are the smallest sized particles, and

they feel "sticky" and they are hard to squeeze.

SOIL TEXTURE

Texture is determined by its % sand, silt, and clay.

Texture determines the type of soil, and the soil’s properties.

Physical Property: Texture

Loam: ideal for

agricultural soil

Predicts soil properties-fertility, etc. Sandy soils

low organic matterPoor retention water and nutrients (it flows through)

As silt and clay fractions increase, soilsMore organic matterBetter bufferedBetter retention of water and nutrients

But you can have TOO much clayHard to cultivate-too sticky when wet; too hard when dryShrinks and swells

The best soils are a mix of all three and are called LOAM

Why care about soil texture?

Check-InSOIL PROPERTY

DEF. WHY CARE?

OTHER INFO

Texture Way a soil feels; Based on sand, silt, and clay amounts

Determines many soil properties; how workable; holding capacity

Sand = largest, silt = middle, clay = smallest;SAND—bad holding capacity; low OM; workable; CLAY- great holding; but shrink/swell; LOAM = 1/3 mix—BEST!!

Physical: Porosity & Permeability again… Think it is important??

Definition?◦ amt of soil pore spaces (aeration)

Porosity

PermeabilityDefinition?

– ability of water to percolate through

____ porosity = ____ permeability

Soil Composition continued…

As water infiltrates the soil, it can carry dissolved materials with it.

Leaching: Removal of dissolved materials from the soil by water percolating downward

Illuviation: Deposition of leached material in lower layers◦ Some substances completely leach out of the

soil because they’re so soluble that they migrate right down to the groundwater.

Physical: Ionic Charge Soil minerals are often present as ions. Minerals ions are either positively or

negatively charged. Clay particles have mostly negative

charges on their outer surfaces.◦Positively charged mineral ions are

attracted to the soil particles and are held for plant use. potassium K+ and magnesium Mg2+

◦Negatively charged mineral ions are repelled by the soil particles and are washed away from roots. nitrate NO3

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Physical Characteristics (review)

Why would 100% sand be undesirable for plants? Why would 100% clay be undesirable for plants? Answer SAND / SILT / CLAY for the following:

◦ A soil with a lot of _____ would have good drainage, but poor nutrient-holding characteristics.

◦ A soil with a lot of _____ would have poor drainage, but good nutrient-holding characteristics.

◦ A soil with a lot of _____ would have low porosity.◦ A soil with a lot of _____ would have high permeability.

Chemical Property: pH

Most soil pH is 4-8. Plants are happiest at 6-7. Plants are affected by soil pH.

(1) The solubility of certain nutrient minerals varies with differences in pH Low pH = Al & Mn are more soluble, and can be absorbed in toxic

amounts. High pH = some salts are less soluble, and are less available to the

plants.(2) Soil pH affects the leaching of nutrient minerals.

Acidic soil = positively charged ions are less soluble and are unavailable for plants.

PESTICIDE TIE-IN: If soil is too acidic, applied pesticides, herbicides, and fungicides will not be absorbed (held in the soil) and they will end up in runoff

Plant pH PreferencesPlants the like

strongly acid soil (pH 4.0 to 4.0)

Plants that prefer slightly acid soil (pH 5.0 to 6.5)

Plants that prefer neutral soil

(pH 6.0-8.0)

• Sundew• Pitcher Plants• Venus flytrap• Azalea• Rhododendron• Camellia

• American holly• Orchids• Many evergreen

trees and shrubs• Strawberries• Potatoes• Carrots• Fescue grasses

• Alfalfa• Bluegrass• Most vegetables (lettuce, tomato)• Grains (corn,

wheat)

Soil Conditioners (Lime) CaCO3 or MgCO3 Soil amendment / conditioner… not a

fertilizer! Important functions:

1. Corrects soil acidity2. Adds important plant nutrients— Ca & Mg3. Reduces solubility & toxicity of certain elements

in the soil Al, Mn, FeThis toxicity could reduce plant growth under acid conditions.

4. Promotes availability of major plant nutrients. Zn, Cu, P

5. Increases bacterial activity & mutualistic relationships.

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Soil stores important nutrients

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Phosphorus (P)Nitrogen (N)

Potassium (K)Calcium (Ca)

Magnesium (Mg)

Dr. Greg Pillar, Queens College

Nitrogen

Phosphorus

Soil Nutrients• Nutrients: essential chemical elements needed for

the growth of healthy plants

• Plants get nutrients from soil after the nutrients have dissolved in the soil solution (the water around soil particles)

• Each nutrient has specific roles in producing healthy plants

FUNCTIONS Used to make chlorophyll Responsible for rapid growth and health

green leavesTOO LITTLE/TOO MUCH Not enough nitrogen? Plant leaves turn yellow Too much nitrogen?

Long weak stems/no floweringMOVEMENT THROUGH and TO THE

SOIL: Nitrogen Cycle

NITROGEN

FUNCTION Responsible for root development and growth Helps plant produce flowers and fruit Important in photosynthesis and respiration, energy

transfer and storageTOO MUCH/TOO LITTLE Not enough phosphorus?

Purplish or deep green leaves Poor root system

Too much phosphorus? No symptoms Plants NEVER have too much phosphorus—there’s not a lot out there—what does exist bonds to soil or leaches away quickly

MOVEMENT THROUGH and TO THE SOIL: Phosphorus cycle

PHOSPHORUS

What is a soil profile?

Picturing a Soil ProfileImagine a hill…

Imagine the bulldozer taking a good chunk out of that hill.Hey, look at all the layers! That’s the soil profile…

Imagine a bulldozer…Layers are called horizons

Soil Profiles◦ the sequence of layers (horizons) from the surface downward

to rock or other underlying material Soil Layers “O”- organic horizons, litter derived from dead

plants andanimals

A- eluvial, mineral horizons which lie at or near thesurface and are characterized as zones of maximumleaching (E = Exit)

B- illuvial, washed in, layer of accumulation (I = Into)

C- unconsolidated material under A&B layers Bedrock

Soil Profiles

Soil profile example

Maryland Soil Profile

Soil Profile Example 2

Florida Soil Profile

Some Review Questions

• What effect does temperature have on soil?– It controls how fast organic material breaks down

• How might pH affect soil?– It changes what nutrients are available and toxins

(e.g. metals) in the soil• What is a soil profile?

– The cross-section of soil that shows different layers